Syria and the Lebanese militant group Hezbollah have rejected US accusations that they are seeking to topple the Lebanese government with Iran's help.
Syria and Iran are supporters of militant group Hezbollah
Damascus denounced US "rumours", while Hezbollah accused the US of using Lebanon as a tool against its enemies.
The White House said on Wednesday it had "mounting evidence" of a plan to unseat the government in Beirut.
The US believes Syria may try to block the formation of a tribunal over the killing of Lebanese ex-PM Rafik Hariri.
Washington did not present any evidence, saying it was classified.
"The latest American position is a blatant interference in a Lebanese internal affair concerning the Lebanese people's choices over their government and policies," Hezbollah said in a statement.
"It is also meant to throw Lebanon into Washington's battle against forces and states that are friendly and brotherly to Lebanon and its people, including Iran and Syria," it said.
Damascus earlier rejected the claims in a foreign ministry statement.
"The rumours put about by the US administration according to which Syria, Iran and Hezbollah are seeking to destabilise the situation in Lebanon are wrong," the statement said.
A Syrian government newspaper described the comments as "pure vilification meant to raise turmoil in Lebanon and cause fallout with Syria."
Hezbollah is backed by Syria and Iran, and has two ministers in Lebanon's government.
Mr Hariri was killed in an explosion in February 2005
The BBC News website's world affairs correspondent Paul Reynolds says the White House statement appeared to result from the tense situation in Lebanon, where Hezbollah is demanding one third of cabinet seats, thereby giving it a veto over decisions.
Such a veto would enable it to block approval of the international tribunal to try suspects in the assassination of Mr Hariri in February, 2005, our correspondent says.
The Hezbollah leader, Sheikh Hassan Nasrallah, has threatened street demonstrations in support of his demand.
The US is concerned that this instability could result in the fall of the government of Prime Minister Fouad Siniora.
Some see it as healthy opposition to the government, but critics of Hezbollah say the group is indeed doing the bidding of Iran and Syria, says the BBC's Kim Ghattas in Beirut.
The claim from the White House came as Britain held its highest level talks with Syria since the 2003 invasion of Iraq.
Prime Minister Tony Blair's foreign policy adviser, Nigel Sheinwald, met Syrian President Bashar al-Assad and senior ministers.
Few details were given of the discussions, but the Financial Times newspaper reported that the visit was aimed at pressing Syria to cease its support for radical groups.
As well as its relationship with Hezbollah, Syria is thought to have influence on some of the insurgent groups operating in Iraq.